Schematic representation of the neural tube of amphioxus 14–18 hr neurulae showing expression of genes involved in the synthesis and/or transport of neurotrasmitters. The neural tube (anterior on top) has been opened such that cells located dorsally in the neural tube are lateral in the diagram. Black circles = location of the frontal eye (anterior) and of the first pigment spot (poterior). S1-6 = position of somites 1–6. FB/MB = forebrain and midbrain; HB = hindbrain. The spinal cord starting at the level of somite 7 is not shown due to the lack of labelled neurons at these stages.